The present invention relates to apparatus for electrostatic spray coating and more particularly to a new and improved muzzle for use in a conventional electrostatic spray gun.
The prior art discloses various types of apparatus for atomizing liquid or pulverized coating material and for the electric charging of the atomized particles. It is a fundamental law of physics that unlike charges attract. Thus, by charging coating material particles to one polarity and an article to be coated to the opposite polarity the coating particles will be attracted to the surface of the article to be coated, and high quality coatings can be achieved.
Typical of a prior art electrostatic spray gun is that shown in U.S. Pat. No. 3,608,823. The spray gun disclosed in the '823 patent comprises a tube of electrically insulating material, the free end thereof supporting an atomizer for the coating material and the other end being connected to a conveying means for supplying coating material by means of a propellant gas stream. A high-voltage generator and electrodes are provided for the electric charging of the coating material and for maintaining a high-voltage field between the gun and the object to be coated. The electrodes are formed as spikes of tungsten successively projecting from the internal wall of the tube in the direction of flow of the propellant gas-coating material mixture. A disadvantage of this type of electrode structure is that the electrodes wear away relatively quickly, degrading performance of the spray gun and requiring replacement.
In U.S. Pat. No. 4,066,041, a nozzle for an electrostatic spray apparatus is provided which has at least 2 electrodes at the tip thereof. High voltage of the same polarity, but of different magnitude, is applied to the electrodes, and a transverse electrical field is generated which covers the nozzle opening. As in the '823 patent, this structure also utilizes delicate electrodes which wear away and need to be replaced at relatively frequent intervals.
U.S. Pat. No. 3,901,184 discloses an electrostatic spray nozzle containing a center conductor and having a bell-shaped deflector end. The deflector is part of the center conductor, and is therefore itself conductive. The center conductor is axially supported within the spray nozzle by an insulated spider.
U.S. Pat. No. 4,011,991 discloses a center electrode which is conductive and which charges coating particles. The center electrode includes a conductive-bell shaped deflector end.
Other patents which show a conductive center electrode in an electrostatic spray apparatus are U.S. Pat. Nos. 2,855,245; 3,774,573; and 3,056,557. None of these patents, however, relate to a muzzle for use in an electrostatic spray gun which can be quickly connected to or disconnected from the spray gun housing.
It has been found that the electrodes in the prior art structures are quite fragile, and as noted above, tend to wear away rather quickly. It would therefore be advantageous to provide a muzzle for use in an electrostatic spray gun which includes a rugged and reliable electrode, in the form of a conductive rod, which is adapted to replace the more fragile nozzle assembly in conventional electrostatic spray guns. Such a muzzle should be easy to install and remove from an electrostatic spray gun, and in order to fit in conventional spray guns, must be capable of coupling power to the center electrode without any need for modification of the spray gun itself. The muzzle must, of course, also be of a structure which efficiently and reliably imparts a proper electrostatic charge to the coating material which exits from the spray gun.
The present invention provides such a muzzle.